In some conventional tire air pressure detection systems, a transmitter including a pressure sensor or the like is directly attached to each wheel, to which a tire of a vehicle is attached, so that the pressure sensor generates a pressure detection signal and the transmitter transmits the detection signal from a wheel. An antenna and a receiver are attached to a vehicle chassis so that the antenna receives the detection signal and the receiver detects the tire air pressure at a chassis (vehicle body).
For detecting a position of the wheel, to which the transmitter is attached, the transmitter is configured as a transceiver device having a receiver function as well as a transmitter function thereby to perform bilateral communications with the receiver of the chassis.
For example, in the following patent document 1, a trigger device attached to a chassis transmits a trigger signal so that a wheel position is determined based on attenuation of the trigger signal. Specifically, a transceiver device attached to the wheel measures a magnitude of the trigger signal and transmits data of the signal magnitude to the chassis. Thus, the position of each wheel, to which the transceiver device is attached, is detected by measuring a distance from the trigger device to the transceiver device.
In the following patent document 2, a trigger device is attached to a vehicle chassis for each of a front wheel and a rear wheel. That is, the number of the trigger devices is less than the number of wheels of a vehicle. A transceiver device attached to a wheel gradually lowers its signal reception sensitivity until it comes to fail to receive a trigger signal of the trigger device, when the trigger signal is output from the trigger device. Thus, the position of each wheel, to which the transceiver device is attached, is determined by measuring a distance from the trigger device based on until when the transceiver device receives the trigger signal.
It is thus possible to detect a position of the wheel, to which the transceiver device is attached, based on the magnitude of the trigger signal received by the transceiver device. However, the transceiver device must be held in the active state (trigger signal wait condition) to receive the trigger signal. Since the electric power of a battery provided at the wheel for the transceiver device is limited, it is not preferred to persistently maintain the receiver function of the transceiver device in the activated state. It is therefore proposed to prolong the life of the battery by periodically turning on and off the signal wait condition of the transceiver device.    Patent document 1: JP 2007-15491A (US 2007/0008097)    Patent document 2: JP 2007-17260A (US 2007/0008097)
If the signal wait condition of the transceiver device is turned on and off periodically, that is, the signal wait is performed intermittently, to prolong the battery life, the signal wait time may differ among a plurality of transceiver devices. In the case of patent document 1, therefore, if one transceiver device fails to receive the trigger signal of the trigger device, it is not possible to determine whether it is caused by the difference of the signal wait time or by a long distance from the trigger device. In the case of patent document 2, it is not possible to determine whether the failure of reception of the trigger signal is caused by the difference of the signal wait time or the lowered sensitivity of the transceiver device. As a result, the wheel position cannot be detected accurately or it takes too much time to complete the detection of the wheel position.